Circulating cholesterol is carried by plasma lipoproteins. Lipoproteins are particles of lipid and protein that transport lipids in the blood. Low-density lipoproteins (LDL) and high-density lipoproteins (HDL) are the major cholesterol carriers. LDL is believed to be responsible for the delivery of cholesterol from the liver to extrahepatic tissues in the body.
The term “reverse cholesterol transport” (RCT) describes the transport of cholesterol from extrahepatic tissues to the liver where it is catabolized and eliminated. It is believed that plasma HDL particles play a major role in the reverse transport process, acting as scavengers of tissue cholesterol. RCT consists mainly of three steps: (a) cholesterol efflux, the initial removal of cholesterol from various pools of peripheral cells; (b) cholesterol esterification by the action of lecithin:cholesterol acyltransferase (LCAT), preventing a re-entry of effluxed cholesterol into cells; and (c) uptake/delivery of HDL cholesteryl ester to liver cells.
High levels of HDL and apolipoprotien A-1 (ApoA1), the major HDL protein, have long been associated with decreased risk for cardiovascular disease. ApoA1 is a single polypeptide chain with 243 amino acid residues of known primary amino acid sequence (Brewer et al., (1978) Biochem. Biophys. Res. Commun. 80: 623-630). ApoA1 acts as an acceptor of cellular cholesterol in the reverse cholesterol transport by mediating cholesterol efflux from cells.
Each HDL particle contains at least one copy (and usually two to four copies) of ApoA1. ApoA1 is synthesized in humans in the form of a preproapolipoprotein of 267 residues by the liver and small intestine which is secreted as a proprotein that is rapidly cleaved by the action of a calcium-dependent protease to generate a mature 243 amino acid polypeptide and secreted into the plasma. Apo A1 has been postulated to possess eight tandem repeating 22 mer sequences and two 11 mer sequences, most of which have the potential to form class A amphipathic helical structures (Segrest et al. (1974) FEBS Lett. 38: δ 247-253). Characteristics of the class A amphipathic helix include the presence of positively charged residues at the polar-nonpolar interface and negatively charged residues at the center of the polar face (Segrest et al. (1974) FEBS Lett. 38: 247-253; Segrest et al. (1990) Proteins: Structure, Function, and Genetics 8: 103-117).
ApoA1 forms three types of stable complexes with lipids: small, lipid-poor complexes referred to as pre-beta-1 HDL; flattened discoidal particles containing polar lipids (phospholipid and cholesterol) referred to as pre-beta-2 HDL; and spherical particles containing both polar and nonpolar lipids, referred to as spherical or mature HDL (HDL3 and HDL2). Most HDL in the circulating population contain both ApoA1 and ApoAII (the second major HDL protein) and are referred to as the A1/AII-HDL fraction of HDL. However, the fraction of HDL containing only ApoA1 (referred to herein as the A1-HDL fraction) appear to be more effective in RCT. Certain epidemiologic studies support the hypothesis that the A1-HDL fraction is anti-atherogenic. (Parra et al., 1992, Arterioscler. Thromb. 12:701-707; Decossin et al., 1997, Eur. J. Clin. Invest. 27:299-307).
The evidence linking elevated serum cholesterol to coronary heart disease is overwhelming. For example, atherosclerosis is a slowly progressive disease characterized by the accumulation of cholesterol within the arterial wall. Compelling evidence supports the concept that lipids deposited in atherosclerotic lesions are derived primarily from plasma LDL; thus, LDLs have popularly become known as “bad cholesterol”. In contrast, HDL serum levels correlate inversely with coronary heart disease, and as such are regarded as a negative risk factor. It is hypothesized that high levels of plasma HDL are not only protective against coronary artery disease, but may actually induce regression of atherosclerotic plaques (e.g. Badimon et al., 1992, Circulation 86(Suppl. III):86-94). Thus, HDL has popularly become known as the “good cholesterol”.